Airbag devices for protecting an occupant by inflating an airbag during a vehicle collision, and pretensioners for removing a slack of a webbing of a seat belt device by rapidly winding up a predetermined length of the webbing are well known. For activating such an airbag device or a pretensioner, it is required to determine the occurrence of a vehicle collision and the magnitude of the vehicle collision (crash severity). The crash severity correlates with the relative velocity to the object and the mass and the rigidity of the object.
Conventionally, the acceleration is detected for determining the magnitude of the collision. For example, in Japanese Patent Unexamined Publication No. 11-78769, the crash severity is determined on the bases of the magnitude of acceleration, the variation with time of the acceleration, and the variation with time of the velocity. In this publication, the variation with time of the velocity is obtained from the values by integrating the acceleration with time. It also should be noted that the protection of an occupant will be sometimes referred to as “restraint”. In addition, acceleration means acceleration (the derivative with respect to time of speed) acting on a vehicle in the decelerating direction during a collision.
A collision determining method by detecting the deformation of an exterior member of a vehicle due to the collision is disclosed in Japanese Patent Unexamined Publication No. 11-78770. Distortion sensors are mounted on the exterior member of the vehicle so as to detect the deformation amount and the deformation speed of the exterior member, thereby determining the magnitude of the collision.
A collision determining method by detecting the deformation speed of a front part of a vehicle during deformation of the vehicle body due to the collision is disclosed in Japanese Patent Unexamined Publication No. 2001-171476. According to this publication, acceleration sensors are disposed at two predetermined locations on a vehicle body so as to detect the compressive deformation speed of the vehicle body between the two sensors, thereby determining the magnitude of the collision.
In the conventional systems, the determination of collision is carried out by signals from an acceleration sensor located in a vehicle cabin. The acceleration sensor placed in the vehicle cabin is suitable for judging the full perspective impact pulse applied to an occupant by the vehicle collision. Since the location is far from the front, the delay in time for collision determination is improved by placing another acceleration sensor at a location near the front.
In a typical airbag device, it takes time from 30 milliseconds to 40 milliseconds to fill an airbag with gas so that the airbag is sufficiently developed. Therefore, it is required to detect the occurrence of a collision and, then, activate an occupant protective device before a large impact is transmitted to the occupant.
In case of a typical sedan, the duration of an impact pulse due to a collision against a full-overlap barrier at a velocity of 50 kilometers per hour (km/h) is on the order of 70 milliseconds to 100 milliseconds (this is the amount of time it takes for the velocity to become zero). Of course, it is necessary to activate the occupant protective device before the occupant collides with a vehicle cabin portion by forward movement. The amount of time for which the occupant moves forward about 10 centimeters (cm) relative to the vehicle body is on the order of 50 milliseconds after collision at an impact velocity of 50 kilometers per hour. For ensuring the airbag to function as an occupant protective device, it is required to judge the occurrence of a collision and output an activation signal within 20 milliseconds after collision. In case of collision at a velocity of 50 kilometers per hour, the crashing stroke of a front end of a bumper is on the order of 13 centimeters after 10 milliseconds from collision and on the order of 25 centimeters after 20 milliseconds from collision. During the first 5 milliseconds after collision, the bumper receives impact so as to create an impact pulse. After 10 m sec., the interference between the impact barrier and the main frame of the vehicle body starts to create a large impact pulse.
Even with a collision at a velocity of 18 kilometers per hour (such a collision does not always need the activation of the airbag), an impact pulse is created because of the crashing of a bumper. The control system is provided with a function to determine whether to activate the airbag within 20 milliseconds after collision, without reacting to such a minor collision, by detecting the deceleration of the vehicle as a whole with the force received by the front end of the bumper.
In the collision determining method by detecting the acceleration disclosed in Japanese Patent Unexamined Publication No. 11-78769, the result of collision determination and crash severity is affected by the structure of a vehicle around a collided portion, the rigidity of the object, and the like.
In the collision determining method disclosed in Japanese Patent Unexamined Publication No. 11-78770, the deflection speed of the exterior member can be detected by the sensors mounted on the exterior member. However, the detected deflection speed is not always the deformation speed of the vehicle body structure in an impact loading direction. In addition, since the deformation stroke of the exterior member is generally short, it is difficult to detect the deformation speed as the collision deformation progresses.
Therefore, it is difficult to determine in an early stage of collision whether the collision is a collision between the vehicle and a small-mass high-velocity object, with which only the exterior member is deformed (a collision not creating such a vehicle deceleration that occupant restraint by a passive safety system is needed) or a medium-velocity barrier collision, with which the vehicle body is largely deformed (a collision creating such a vehicle deceleration that occupant restraint by a passive safety system is needed). That is, it is difficult to determine the occurrence of a collision and the crash severity from the deformation speed of the exterior member.
The collision determining method disclosed in Japanese Patent Unexamined Publication No. 2001-171476 is not a method of detecting the deformation speed of the vehicle front end portion. As one of the acceleration sensors is located at the vehicle front end, an excess impact over the rated value of the sensor is exerted on the sensor in an early stage of a collision so that the mounting portion of the sensor is deformed, thus shifting off the detection axis of the sensor. With the shifted detection axis of the sensor, the deformation speed of the vehicle front end can not be properly detected.
The systems mentioned in the above have a number of shortcomings. For example, there is poor stability in collision determination. It is desired to activate a restraint device at a proper timing for every type of collision. However, the configuration, the rigidity, and the velocity of a collided object actually differ from collision to collision. For instance, in cases where the configuration of a collision barrier is offset or center pole, the impact at the early stage of collision is small so as to delay the collision determination unnecessarily. In cases of pole collision, the impact at the early stage is small, but a large impact is created after that. In this case, the collision delay determination degrades the restraint performance.
Since an airbag is filled with high-pressure and high-temperature gas for 30 milliseconds by igniting powder, the airbag has large energy during deployment. During the deployment, if the airbag collides with an occupant, a large force is exerted on the occupant. To reduce this result, it is desired to shorten the time for the collision determination so as to lengthen the time to be used for deployment of the airbag, thereby reducing energy of gas for deployment. In the conventional sensor system, to prevent the restraint device from activating due to a large impact pulse created by a low-grade collision, running on a rough road, or bumping of the engine against a curb, the time for the collision determination should be on the order of 15 milliseconds that is longer than the duration of the pulse. The technology for shortening the time almost reaches the boundary.
In addition, there is difficulty in determining the crash severity. Occupant protective devices have been improved to increase the safety for every person having a large body or a small body. An airbag with high pressure is needed for sufficiently restraining an occupant having a large body against a sever collision at high velocity. On the other hand, airbag with high pressure is not needed for softly receiving an occupant having a small body in case of a collision at moderate velocity. To satisfy this, a passive safety system which has a choice between two levels in controlling the amount of gas to be supplied into the airbag has been introduced. If the crash severity can properly be determined at an early stage by a collision detecting means, the pressure of the airbag can be controlled according to the crash severity, thereby further improving the performance of the occupant protective device. Therefore, it is desired to provide a high-precision detecting means.
Improved body structures for controlling the crash property has been employed. The structure of the front end is configured to absorb energy when encountering a predetermined force. Even with any crash severity, this configuration mitigates the deceleration acting on the vehicle body at an early stage of collision. Therefore, it is difficult to determine the crash severity with high precision at an early stage of collision by the conventional means, because there is no large variation in impact pulse at the early stage.